There are occasions when a ship is towed by another ship. Typically it happens when an icebreaker tows a ship. Towing by icebreaker must be done so that the bow of the ship to be towed has been pulled tightly into the close contact with the stern of the icebreaker. The ship to be towed will be kept in the right place by a towing winch located aboard on the icebreaker. The winch pulls strongly at the wire rope the bow of the towed ship against the stern of the icebreaker during the towing operation.
For towing purposes the stern of the icebreaker must be formed in a special way. At the stern of the icebreaker there is an inwards curved towing notch which makes possible to tow another vessel. Only when equipped with this notch it is possible for the icebreaker to tow a ship in the way that the bow of the ship to be towed is pressed directly towards the stern of the icebreaker.
There are also other kind of special needs for the stern form of a ship. In open water for example the anchor handling and the cable laying operations require different kind of stern forming. At the stern of a supply vessel there must not be any formed towing notches. Instead of that it is on the contrary required that the stern of the supply vessel is straight and the stern roller is installed. In the supply vessel the different kind of hoisting operations are done over the transom with a winch or crane.
The special requirements said above cause that a ship constructed for one purpose can not be used for another purpose. The same ship can not be used for the towing in winter and for supply operations in summer. The requirements for the stern shape and the stern structure are so different in winter operations and respectively in summer operations that it is not possible to build a fixed structure which would be suitable for the both purposes.
The problem with icebreakers, however, is their short operating time in winter. It should be able to use icebreakers also in the summertime in open water. Correspondingly the supply vessel can not be used for towing in wintertime because of its unsuitable stern construction.